1. Field of the Invention
The subject invention relates to a knee joint prosthesis, and particularly a posterior stabilized replacement knee joint prosthesis.
2. Description of the Related Art
A natural knee joint includes the distal end of the femur with articular cartilage, the proximal end of the tibia with articular cartilage and a meniscus between the femur and tibia. The femur and the tibia are held in a proper relationship to the bearing by ligaments. These stabilizing ligaments include the posterior cruciate ligament, the anterior cruciate ligament and collateral ligaments.
Flexion of the knee causes the tibia to rotate relative to the femur about an axis that extends generally in a medial-to-lateral direction and simultaneously causes the contact area of the femur to roll back relative to the tibia. Flexion also generates rotation of the tibia about its own axis. The amount of rotation of the tibia during flexion of the knee is controlled and limited by the ligaments.
The natural knee joint can become damaged or diseased. For example, damage or disease to the knee can deteriorate the articular surfaces of the femur or tibia and can damage the articular cartilage between the bones. The prior art includes prosthetic knee joints to replace a damaged or diseased natural knee. A prosthetic knee joint typically includes a femoral component that is mounted to the distal end of a resected femur, a tibial component mounted to the proximal end of a resected tibia and a bearing between the femoral and tibial components. The inferior face of the femoral component of a prosthetic knee joint typically defines a pair of arcuate convex condyles. The superior face of the bearing has a corresponding pair of arcuately concave regions for articular bearing engagement with the condyles of the femoral component. The superior face of the tibial component may be substantially planar and is disposed in engagement with the inferior face of the bearing.
Prior art prosthetic knee joints have taken many different forms, depending upon the preferences of the orthopedic surgeon, the condition of the natural knee and the health, age and mobility of the patient. Some prior art knee joint prostheses fixedly secure the inferior surface of the bearing to the superior surface of the tibial component. Other prior art knee joint prostheses permit rotational movement between the bearing and the tibial component. Still other prior art knee joint prosthesis permit a controlled amount of anterior-posterior sliding movement between the bearing and a tibial component. Movement of the bearing relative to the tibial component achieves many functional advantages, as described in the prior art. Prior art knee joint prostheses that incorporate certain of the structural and functional features referred to above are disclosed in U.S. Pat. Nos. 4,470,158 and 4,309,778.
As noted above, the inferior bearing surface of the femoral component on most prosthetic knee joints comprises a pair of convexly arcuately condyles. The condyles of the femoral component are in articular bearing engagement with arcuately concave regions on the superior face of the bearing. Thus, the superior face of the bearing typically includes a pair of dished regions each of which has a relatively depressed center portion and a relatively elevated peripheral lip. As explained above, flexion of the knee joint causes the tibia to rotate about a medial-lateral axis relative to the femur. Flexion also causes the tibia to rotate around its own axis. These combined movements may cause the condyles of the femur to ride up or climb the concavities on the superior surface of such a bearing and to approach the peripheral lips of the bearing unless the bearing moves with the femur. Thus, flexion tends to move the relative components of the prosthetic knee toward dislocation. The degree to which dislocation is possible depends on several factors, most significantly, the presence or absence of ligaments and the mobility and configuration of the bearing. The likelihood of dislocation also depends upon the degree of flexion and on the degree of congruency between the inferior articular bearing surface of the femoral component and the superior surface of the bearing. For example, climbing of the femoral component on the bearing is not a significant problem even in fixed bearing knees that have a substantially flat superior surface on the bearing. However, the relatively great incongruency between the inferior bearing surface of the femoral component and the superior surface of the bearing on these prosthetic knees results in a very high contact stress that can damage the bearing. Fixed bearing prosthetic knees that have greater congruency between the femoral component and the bearing provide desirably low contact stress. However, the greater congruency creates the problem of the femoral component climbing on the bearing during flexion or rotation, and hence creates the potential of dislocation.
Valgus-varus stability of a knee joint refers to the ability of the joint to resist the lateral forces or rotary forces that would cause rotation of the tibia relative to the femur in the frontal plane. Lateral forces or rotary movements that cause rotation of the tibia relative to the femur in the frontal plane tend to create a dislocation. Such dislocation is particularly likely to occur on either the medial or lateral side of the prosthesis, depending upon the direction of the lateral forces. Such a dislocation in a prior art prosthesis is shown in FIG. 18 hereto.
The prosthetic knee joint is under a compressive loading during normal activities. As a result, valgus-varus moments typically are resisted adequately by the articulating surfaces of the prosthetic components and by the ligaments. However, there are instances where additional valgus-varus stability may be desired, such as those instances where ligaments are deficient.
Some prior art prosthetic knee joints enhance valgus-varus stability by providing a stabilization post that extends into a posterior region between the femoral condyles. This region would be occupied by the posterior cruciate ligament if that ligament were present. Climbing of the femoral component on the bearing also is a particular problem for prosthetic knee joints that employ a posterior stabilization post. In particular, the climbing of the femoral component on the bearing substantially increases shear forces on the post and can lead to traumatic failure of the prosthesis.
Prosthetic knee joints that permit anterior-posterior sliding movement of the bearing on the tibial component provide superior roll back. In this regard, the term xe2x80x9croll backxe2x80x9d refers to a posterior movement of the contact point of the femur relative to the tibia during flexion. Roll back is a potentially desirable feature of a prosthetic knee, as explained below. However, roll back, in prior art prostheses often has caused the femoral component to climb on the bearing, and thus has increased the probability of dislocation.
A prosthetic bearing that can slide posteriorly during flexion reduces impingement between the bearing and anterior soft tissue of the knee. Thus, a prosthetic knee joint with a bearing capable of anterior-posterior sliding movement can reduce discomfort during deep flexion.
A prior art prosthetic knee joint with a stabilizing post and a bearing capable of anterior-posterior sliding movement is shown in U.S. Pat. No. 5,395,401 which issued to Bahler. In particular, U.S. Pat. No. 5,395,401 shows a prosthetic knee having a tibial component and a bearing slidably disposed on the superior face of the tibial component. The inferior surface of the bearing is provided with a dovetailed groove that extends along an anterior-posterior direction at a location between the two concave condyles formed on the superior surface of the bearing. The bearing shown in U.S. Pat. No. 5,395,401 also includes a notch extending into the posterior portion of the bearing at a location between the two concave condyles of the bearing. The notch registers with the dovetailed groove of the bearing. The prosthesis of U.S. Pat. No. 5,395,401 further includes a control arm formed from a metal alloy and having a pin that is pivotally engaged in a recess formed on the tibial component. The control arm includes a dovetailed portion that slidably engages in the dovetailed groove on the inferior surface of the bearing. The control arm shown in U.S. Pat. No. 5,395,401 also has a stabilizing projection that extends through the notch in the bearing and between the condyles of the femoral component. The post is dimensioned for insertion into a box-like structure formed in the femoral component between the two convex condyles of the femoral component. The box-like structure has side walls that engage the stabilizing projection. This would appear to require metal-to-metal sliding contact, which is undesirable. Other embodiments of U.S. Pat. No. 5,395,401 show lateral walls of the bearing that extend into the box-like structure and that lie between the stabilizing projection and the side walls of the box-like structure in the femoral component. This would avoid metal-to-metal sliding contact, but would require a very complex bearing. Nothing in U.S. Pat. No. 5,395,401 would prevent dislocation of the femur from the bearing.
The prior art includes other prosthetic components that have posterior stabilizing posts that extend unitarily from the bearing and into the space between the femoral condyles. Prior art prosthetic joints of this type are shown, for example, in U.S. Pat. Nos. 5,658,342; 5,489,311; 5,330,534; 4,950,298; 4,888,021; 4,634,444 and 4,568,348. All of these prior art prostheses are used for joint replacements where the posterior cruciate ligament cannot be retained or is deficient. Additionally, most of these prior art prosthetic components are for use when both collateral ligaments can be retained.
Despite the various attributes of the prior art prosthetic components, it is desired to provide a prosthetic knee joint that provides superior dislocation resistance than other non-hinged prosthetic knee joints.
It is another object of the subject invention to provide a prosthetic knee joint that avoids any significantly likelihood of dislocation while simultaneously permitting anterior-posterior sliding movement of the bearing relative to the tibial component.
It is a further object of the subject invention to provide a prosthetic knee joint with enhanced dislocation resistance and superior roll back.
Still another object of the subject invention is to provide a prosthetic knee joint that provides enhanced dislocation resistance and reduced shear on a posterior stabilization post.
A posterior stabilized prosthetic device is provided. The prosthetic device includes a femoral component for mounting to the distal end of a resected femur, a tibial component for mounting to the proximal end of a resected tibia, a bearing disposed between the femoral component and the tibial component and a control arm assembly mounted between at least portions of the bearing and the tibial component.
The bearing of the subject prosthetic joint includes a superior surface having a pair of concave condyles configured for articular bearing engagement with the femoral component as explained further below. The bearing further includes an inferior surface configured for sliding bearing engagement with the superior surface of the tibial component. The inferior surface of the bearing is further characterized by a dovetail grove extending in an anterior-posterior direction. The posterior extreme of the bearing includes a notch or slot centrally disposed between medial and lateral extremes of the bearing and extending continuously from the superior surface to the inferior surface. The notch is substantially aligned with the dovetail groove in the inferior surface of the bearing.
The control arm assembly includes a conical bearing configured for rotary bearing engagement in a conical recess formed in the tibial component. The control arm further includes a dovetailed guide at the superior large diameter end of the conical bearing. The dovetailed guide is dimensioned for sliding movement in the dovetail groove of the bearing and includes opposed anterior and posterior ends that may be spaced from the conical bearing. A post projects superiorly from the posterior end of the dovetailed guide. The post defines a medial-lateral width that permits slidable engagement of the post in the notch of the bearing. Portions of the post spaced from the dovetailed guide include medial and lateral projections. The projections preferably have arcuately convex inferior cam surfaces. The control arm may be formed from a plurality of separate components and from a plurality of different materials. For example, superior portions of the post that have the medial and lateral projections may be formed from plastic. This plastic superior portion of the post may be joined non-rotatably to the remainder of the control arm by a separate metallic support pin that extends upwardly from the dovetailed guide through inferior portions of the post and into the superior portion of the post that has the projections formed thereon. The dovetailed guide preferably is formed from metal and conical bearing preferably has at least outer surfaces formed from plastic. This multi-part construction avoids metal-to-metal or plastic-to-plastic wear.
The femoral component has a superior surface configured for secure mounting to the resected inferior end of the femur. The femoral component further includes an inferior surface defined by a pair of arcuately convex condyles configured for articular bearing engagement with the superior surface of the bearing. A notch or cavity extends into the posterior end of the femoral component. A pair of parallel walls on the femoral component define a width for the notch that is substantially equal to the width of the post of the control arm at portions of the post having the projections thereon. The notch is further characterized by a pair of flanges extending toward one another adjacent the inferior surface of the femoral component. The flanges include a pair of opposed parallel edges that are spaced from one another by a distance slightly greater than the width of the post of the control arm at locations below the projections on the post. The flanges further have concave superior cam that engage the inferior cam surfaces of the post during at least certain ranges of flexion.
The prosthetic device of the subject invention enables the bearing to translate posteriorly in response to flexion and also allows axial rotation of the bearing relative to the tibial component. Additionally, superior portions of the post of the control arm are effectively trapped in the notch or cavity of the femoral component. Consequently, the femoral component cannot be dislocated from the bearing. Still further, the post provides valgus-varus stability that is not present in many prior art knee joint prostheses. Such stability is useful where one or both of the collateral ligaments are compromised.
The posterior translation of the bearing in response to flexion is advantageous since rollback of the femur on the tibia can be accomplished without the femoral component climbing up the posterior articulating surface of the bearing. Such xe2x80x9cclimbxe2x80x9d, which would occur with roll back in the prior art prosthetic component, produces additional shearing forces on the post. Climb also increases the compression force on the bearing. However, these problems are avoided with the prosthetic device of the subject invention.